Strings (and arrows) oscillate quite a bit during the shot so a tight-fitting nock can't be good for arrow flight. Over the years I've used both heat (boiling water) and sanding (files, sandpaper or emery boards) to custom-fit nocks. For me, careful/even sanding has proven to be MUCH more reliable, with less chance of weakening the plastic than heating provides. I no longer use heat unless I'm simply trying to remove a broken nock without damaging the shaft. Good question, though.